pwclient: use argparse's error() function for bad input

pwclient: use argparse's error() function for bad input

This reduces the boilerplate we need and provides a more consistent help
output. e.g.:

   $ pwclient update -s FOO -c 1337 314159 1234567
   usage: pwclient update [--help] [-h HASH] [-p PROJECT] [-c COMMIT-REF]
                          [-s STATE] [-a {yes,no}]
                          [ID [ID ...]]
   pwclient update: error: Declining update with COMMIT-REF on multiple IDs

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Stephen Finucane <stephen.finucane@intel.com>